Probing nano-swimmer dynamics via the motion of large tracers

Sub-micron nano-swimmers are ubiquitous in biology and nanotechnology but are extremely challenging to measure due to their small size and driving forces. A simple method is proposed for probing the elusive physical features of nano-swimmers by observing their effect on the motion of much larger, easily traceable particles. Modeling a suspension of swimmers as hydrodynamic force dipoles, we analytically compute a direct, easy-to-calibrate relation between the enhancement of tracer particles diffusion and physical features of the nano-swimmers, namely their force dipole moment and persistence time.